The proposed study is directed toward evaluation of megakaryocytopoiesis, kinetic properties of megakaryocyte precursor cells including the pluripotential stem cells, and homeostatic control of platelet production. Stimulation of this cell system will be produced by peripheral immunothrombocytopenia and its effect on stem cells and on sensitivity of the system to radiation and cytotoxic drugs will be observed in mice and rats. Megakaryocytic abnormalities in genetically anemic mice of the W/WV and S1/S1d strains will be further evaluated in the "normal" steady state, and the effect of perturbations of thrombocytopoiesis and of cell and tissue transplantation will be observed. A bio-assay system for thrombopoietin will be evaluated using sublethally irradiated mice and radioisotopic labeling of platelets.